Verified Answer :

iloveonedirection

Similes compare two things. Because clowns are considered by most to be very funny, the object would also have to be something funny. 
Therefore you could say...

A clown is as funny as a medieval joker.
A clown is as funny as a cat barking like a dog.
A clown is as funny as a comedy movie.

a similes for that is A clown is as funny as my sister falling
or a clown is a funny as a dog barking you really have no wrong answer with that

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